Conventionally, capacities of various types of optical disks have been increased by reducing a volume of information written on tracks of the disk and also reducing a size of a focusing spot on a focal plane through reduction of a wavelength of a laser light used in recording and reproducing and employment of an objective lens having a large numerical aperture.
For example, in a compact disc (CD), a thickness of a disk substrate serving as a light transmission layer (transparent protective layer and space layer provided on an information recording layer, which is also referred to as a transparent substrate) is approximately 1.2 mm, a laser light wavelength is approximately 780 nm, a numerical aperture (NA) of an objective lens is 0.45, and a capacity is 650 MB. Further, in a digital versatile disc (DVD), a thickness of a disk substrate serving as a light transmission layer is approximately 0.6 mm, a laser light wavelength is approximately 650 nm, an NA is 0.6, and a capacity is 4.7 GB. For example, a DVD is used as a disk having a thickness of 1.2 mm by bonding two disk substrates having a thickness of approximately 0.6 mm to each other.
Further, in a blu-ray disc (BD) which has a higher density, an optical disk in which a thickness of a light transmission layer is reduced to 0.1 mm is used such that a laser light wavelength is approximately 405 nm and an NA is 0.85, to thereby achieve a capacity as large as 25 GB per layer.
In addition, there is a high definition digital versatile disc (HDDVD) achieving a large capacity of 18 GB or more by using an optical disk in which a thickness of a disk substrate serving as a light transmission layer is set to 0.6 mm as in a DVD such that a laser light wavelength is approximately 405 nm and an NA is 0.65.
An effective way for achieving a larger capacity in an optical disk is increasing layers of the optical disk. So far, as to a DVD, a dual-layer DVD-ROM, a dual-layer DVD-R and the like have been achieved. In the same manner, dual-layer has been achieved as a technique of increasing a capacity of a BD disk and an HD-DVD disk, and further, there has been progress in the research and development of a multilayer optical disk including 6 layers or 8 layers.
In increasing layers of an optical disk, it is desired to reduce an interval between information recording layers, that is, a distance between layers as much as possible to reduce a spherical aberration which is generated due to a change in thickness of a transparent layer in a case of an inter-layer jump. However, if a distance between layers is made excessively small, there arises a problem that a stray light from an information recording layer other than one for reproducing and recording leaks in, which deteriorates a reproduction signal. The above-mentioned leak-in of the stray light is also referred to as inter-layer crosstalk.
There are proposed several techniques for solving the problem of the above-mentioned inter-layer crosstalk.
The technology described in Patent Document 1 is the extraction optical system for separating and extracting the stray light and reproduction light with a +λ/4 phase plate and a −λ/4 phase plate. Here, the +λ/4 phase plate or −λ/4 phase plate is a birefringent optical element which changes a polarization state of an incident light emerged from a linearly polarized light into a circularly polarized light. Assuming that the +λ/4 phase plate provides light with right optical rotation, the −λ/4 phase plate is defined to provide left optical rotation, and thus a +λ/4 or −λ/4 phase difference is provided to an electric field component of a light parallel to a principal axis direction of birefringence.
However, the extraction optical system of Patent Document 1 is configured to temporarily focus a reflected return light from a multilayer disk with a light focus lens and separate and extract a reproduction light at a position where it turns into a divergent light after reaching a focal point thereof. Therefore, a light receiving device for receiving a reproduction light to generate a reproduction signal is required to collect light again for collecting the reproduction light therein.
In a case where the above-mentioned extraction optical system is applied to an optical head device, it is difficult to configure the extraction optical system without extending an optical path of an optical system for detecting a focus error signal and a tracking error signal which is required in the optical head device.
In order to solve the above-mentioned problem, in the technology described in Patent Document 2, there is configured the optical path systems for detecting a focus error signal and detecting a tracking error signal in the optical head device for removing a stray light from a multilayer disk by disposing a reflective plate which reflects a focused light beam at the focal point for folding the optical path where light is temporarily focused, and also disposing a phase control element in the folded optical path.
However, the folded optical path complicates the optical configuration of the optical head device, and further, loss in quantity of light is generated by an amount by which light passes through optical components disposed in the folded optical path, which is disadvantageous in signal S/N.
In the technology described in Patent Document 3, a return light is separated into two semicircular focused light bundles, and the semicircular focused light bundles are received by light separating and receiving devices which are respectively disposed at focal point positions thereof. In addition, output signals of two light receiving elements of the light separating and receiving devices are selected in accordance with a stray light from the layer which is on the side far from the reproducing layer or a stray light which is on the side close thereto with respect to the optical head device, to thereby obtain a reproduction signal. Accordingly, it is possible to configure the optical path systems for detecting a focus error signal and detecting a tracking error signal at the same time in the optical head device.
However, in a multilayer disk of three or more layers, information recording layers are adjacent to both sides of a reproducing layer in some cases, which causes a problem that the reproduction light cannot be separated and extracted from the stray light.
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-252716
Patent Document 2: Japanese Patent Application Laid-Open No. 2007-133918
Patent Document 3: Japanese Patent Application Laid-Open No. 2005-228436
In the conventional extraction optical system or the optical head device in which the conventional extraction optical system is mounted, it is difficult to easily configure the extraction optical system and the optical system for detecting a focus error and a tracking error at the same time. In addition, considering the configuration of the extraction optical system, it is difficult to detect those from light receiving patterns provided in the same light receiving device.
Therefore, the extraction optical system is required to be configured separately from the optical system for detecting a focus error and a tracking error, and accordingly the number of components is large, which complicates the optical configuration.